1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a hot electron transistor (HET).
2. Description of the Prior Art
Recently, enthusiastic research and development have been directed to an HET as an ultra-high speed element. An HET operable in a room temperature is disclosed in, for example, Applied Physics Letters Vol. 51, No. 13, pp 984-986, September 1987, which is constituted by disposing an emitter barrier layer 2 made of A.lambda.Sb.sub.0.92 As.sub.0.08 and a collector barrier layer 3 made of GaAs at opposite sides of a base layer 1 made of InAs respectively so as to sandwich the base layer 1 therebetween, as shown in FIG. 1 which is a band model diagram of its conduction band in a biasing state.
Since an offset gap .DELTA.E.sub.B-C of a conduction band between the base and the collector of the HET thus constructed is 0.8 eV, it is required that an offset gap .DELTA.E.sub.B-E of a conduction band of the emitter barrier layer between the base and the emitter is to be selected larger than the offset gap .DELTA.E.sub.B-C. Accordingly, the offset gap .DELTA.E.sub.B-E of 1.3 eV is established for the HET. However, with the gap .DELTA.E.sub.B-E between the base and the emitter being such a high value of 1.3 eV, an energy of a hot electron injected from the emitter into the collector is too high to neglect the ionization in the collector due to the hot electron. More specifically, the hot electrons of high energy injected in the collector eject electrons from a valence band in the collector to generate an avalanche current, which degrades the saturation characteristics of the transistor. Hatched portions in FIG. 1 show an energy level where electrons exist.
In the transistor constructed as described above, a layer adjacent to the base layer 1 of In-system, that is, the emitter barrier layer 2 is made of A.lambda.-system, which causes a difficulty in setting a selection ratio of etching between the layers of the A.lambda.-system and the In-system. Accordingly, upon leading a base electrode from the base layer 1 of the In-system which forms this kind of HET in actual practice, there arises a problem that an etching process for partially removing the Al-system emitter barrier layer or emitter layer on the base layer 1 in order to partially expose the base layer 1 is extremely difficult.